Phase behavior of grafted chain molecules: Influence of head size and chain length

TL;DR

This study uses Monte Carlo simulations to explore how head size and chain length influence phase transitions and structures in monolayers of amphiphilic molecules, revealing complex phases including a striped superstructure.

Contribution

It introduces a coarse-grained off-lattice model to analyze phase behavior of grafted chain molecules, highlighting the impact of head size and chain length on phase diagrams and structures.

Findings

Identification of various phases with tilt and positional order.

Discovery of a modulated striped superstructure due to competing length scales.

Effect of chain length on shifting phase boundaries to higher temperatures.

Abstract

Constant pressure Monte Carlo simulations of a coarse grained off-lattice model for monolayers of amphiphilic molecules at the air/water interface are presented. Our study focusses on phase transitions within a monolayer rather than on self aggregation. We thus model the molecules as stiff chains of Lennard-Jones spheres with one slightly larger repulsive end bead (head) grafted to a planar surface. Depending on the size of the head, the temperature and the pressure, we find a variety of phases, which differ in tilt order (including tilt direction), and in positional order. In particular, we observe a modulated phase with a striped superstructure. The modulation results from the competition between two length scales, the head size and the tail diameter. As this mechanism is fairly general, it may conceivably also be relevant in experimental monolayers. We argue that the superstructure would be very difficult to detect in a scattering experiment, which perhaps accounts for the fact that it has not been reported so far. Finally the effect of varying the chain length on the phase diagram is discussed. Except at high pressures and temperatures, the phase boundaries in systems with longer chains are shifted to higher temperatures.